Pale wings a quarter’s diameter across flitted above the swirling surface of Yellowstone National Park’s Firehole River, and underneath a brown trout did a feeding dance. It held steady in the water’s sway and swished its tail to and fro like a two-step dancer toeing to the beat of a tune.

Suddenly, the music played faster. One of those mothlike bugs came down the current, and the trout threw its back into the song. It angled toward the sky, pulled its pectoral fins close, and dropped its jaw to reveal a gaping white gullet. The spotted tail went from slow, slow to quick, quick! Splash.

The caddis hatch has arrived. Craig Mathews, a genial, silver-haired fount of Yellowstone fishing advice, watched the dance from the far bank. He stood at Muleshoe Bend, a bow in the Firehole just upstream from the audacious oranges, yellows, and 200-degree teals of Grand Prismatic Spring. He was 10 miles south of where the Firehole meets the Gibbon and becomes the mighty Madison River. He was also 25 miles from Blue Ribbon Flies, the shop he opened in West Yellowstone the 1970s, now in a wood- sided building painted the color of Baetis wings.

Thousands of hopeful fly fishers drop by every summer to ask Mathews questions, among the most common being, “What patterns are catching trout today on the Firehole and the Madison?”

That day, Mathews didn’t know. For about six years, he had been perplexed by new clouds of thousands of white caddisflies zigzagging in the air above Firehole and Madison rivers. He’d seen a few of the insects in the 1980s and 1990s, and knew they were from the genus Nectopsyche—a large group of caddisflies that thrive in warm water. There were never enough of them to matter much to trout. It made sense that Nectopsyche could find a toehold in Yellowstone rivers, where icemelt runoff collided with geyser overflow hotter than fresh coffee.

But these new clouds of Nectopsyche were too big to be ignored, especially by trout like that tail-wagging brown across the river. Entomologically speaking, something monumental had happened. A once-minor caddisfly’s population had exploded, and it had become one of the most important trout foods on two of Yellowstone’s most famous trout streams.

“It’s truly become the major caddis in those rivers,” Mathews said.

There are at least 50 species of Nectopsyche caddis, entomologists say, and most of them are clustered in Central America and South America, where some are marked with striking gray and brown lines running the length of their wings.

Around a dozen Nectopsyche species combined live in Mexico, the United States, and southern Canada. Like their southern counterparts, they have extra- long antennae, but their wings and bodies are pale, hence their common name: White Millers.

Dr. Oliver Smith, an entomologist at the Smithsonian Institution in Washington, D.C., remembers studying White Millers in central Mississippi by going to the bank of a stream at night with a blacklight.

“They were so thick it looked like a snowdrift, like a glacier running down the water,” he said. “Billions of them.”

Dr. Ralph Holzenthal, a professor of entomology at the University of Minnesota, recalls similar scenes on a few of his state’s 10,000 lakes. James D. Haddock, a biologist at Purdue University in Indiana, wrote the definitive treatment of Nectopsyche in 1977. He notes that White Millers tend to prefer warm, lowland rivers and lakes, and they can thrive in silty water.

The reason White Millers suddenly became so prevalent in Yellowstone’s Firehole and Madison rivers is a source of some debate. While noting that the White Millers prefer warm water, of which there is plenty courtesy of geysers, Smith and Holzenthal believe the cause for the population boom may be less direct. Perhaps an increase in the aquatic vegetation that White Millers eat as nymphs. Or a reduction in predators that eat White Millers.

Whatever the cause, Holzenthal notes that, “When one species begins to dominate a fauna, it is usually not a good thing.”

John Juracek, Mathews’s former partner at Blue Ribbon Flies, watched White Millers overwhelm tan, gray, and green Hydropsyche, an insect that was once the most prevalent caddisfly on the Firehole and Madison rivers. Hydropsyche are now not nearly as important as they once were.

Juracek suspects that water temperature changes lie at the root of the White Miller takeover. Seismic activity underneath Yellowstone has altered geyser flows throughout the millennia, and Juracek believes that more hot water is pouring into the Firehole now than at any time during the last 30 years. For example, Iron Spring Creek, a major tributary to the Firehole River, used to be a coldwater refuge for trout in midsummer, but now its waters run bathtub warm, owing to changes in thermal features along its length. Environmental change, he notes, is probably also a factor.

Regardless of the reason, the White Miller bloom has presented new opportunities for Yellowstone anglers.

“This is—for all intents and purposes—a new emergence, and I don’t think you get to see that in a lot of places; it’s a cool thing to witness,” Juracek said. “This is a fly that has become the most important caddis on the Firehole and the Madison.”

Matching the Hatch
During White Miller hatches, Firehole River trout explode through the surface, making it appear as if the river itself is boiling, just like the hot springs on her banks. Resident trout in the Madison River also stuff themselves on White Millers, as do—on occasion—enormous rainbows and browns that live in Montana’s Hebgen Lake and migrate into Yellowstone Park in the spring and fall to spawn.

White Millers have extra-long antennae and wings about twice the length of their bodies, but don’t be fooled into using imitations that are too large to match the size 18 bodies.

Key times to find White Millers are from Yellowstone’s season opener in May through the end of June, then again in mid-August through the last day of the season in November. Their hatches are thickest in the morning, Juracek said, with a smaller hatch in the evening often coinciding with a massive egg-laying flight.

During these flights, thousands of adults skate over the surface of the water laying eggs. In these situations, Firehole trout breach straight out of the water as they chase egg-laying adults. In the mornings you’ll often see more subtle head-and-tail rises as the trout focus on emergers in the surface film.

X-Caddis and especially the Iris Caddis, Blue Ribbon specialties, are the go-to patterns to imitate White Millers, Juracek said. Variations on these patterns include emergers tied with loops of white nylon representing the gas bubbles caddisfly pupae generate and ride to the surface to hatch.

Juracek said he likes to dead-drift Iris Caddis over specific rising trout. Swinging soft-hackle wet flies can be another effective way to take trout during White Miller emergences.

“Swinging soft-hackles is easy for most people, because they can feel the take,” Juracek said. “Dead-drifting works well for that also, but it’s harder to detect the take.”

Other effective techniques include twitching White Miller cripple patterns in the surface film to imitate the behavior of a caddisfly trying to shake off its pupal shuck. Skating White Miller X-Caddis imitations can also entice acrobatic strikes.

“You see the fish jump out of the water all the time chasing White Millers,” Juracek said. “And they’re pretty good at catching them.”

There is one final piece to the White Miller puzzle, one that Mathews learned when he cast to that dancing brown on a crackling fall day years ago.

The naturals flitting in the autumn breeze had size 14 wings, so Mathews used a size 14 Iris Caddis with a pale green body. The brown trout two-stepped toward the offering, but didn’t commit. Mathews then tried a size 14 X-Caddis, tied with sparkling wisps of Zelon near the hook’s curve to imitate a caddisfly breaking free of its pupal sheath. Again the fish swished up to the beat, but pulled away before contact.

The trout kept dancing, just not with Mathews. A lot rested on his next proposition.

“I knew that if I could unlock the secret to what that trout was eating, it would mean success for years to come,” Mathews said.

He captured one of the naturals and examined it closely. Yes, its wings were a size 14. But look at that pale-green body, behind those oversize antennae. It was smaller. Maybe two sizes smaller.

Mathews rummaged through his fly box and found a size 18 Iris Caddis. With the analytical mind of a scientist he fastened it to his line. With the grace of someone who has studied more than one trout folk dance, he cast toward that Firehole brown.

Slow, slow, quick, quick. Splash!
The dancing trout twisted and flipped, shimmied and shook all the way to his net, its lip pierced by his tiny imitation. He unhooked the brown trout and released it, and then examined the fly.

Here was the key that would open the door to the future of fishing caddisflies on Yellowstone’s Firehole and Madison rivers. Don’t imitate the wing size, Mathews thought, imitate the body size.

“The light bulb finally went on because that one fish had so intrigued me,” Mathews said. “And once we figured out what it took, we started fooling almost every fish.”

That’s a big lesson to learn from any fish. Must’ve been a big trout, right?

“It was nine and a half inches,” Mathews said.

Nate Schweber is the author of Fly Fishing Yellowstone National Park: An Insider’s Guide to the 50 Best Places (Headwater Books, Stackpole Books, 2012).

GALLERY: X Caddis

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<h2>Tying The X Caddis</h2>

<h2>Tying The X Caddis</h2>

<h2>Tying The X Caddis Step 1 of 7</h2>Start the thread two eye lengths behind the eye and wrap back to the bend. Return the thread to the starting point.

<h2>Tying The X Caddis Step 2 of 7</h2>Tie in a hank of zelon and wrap back over it to the bend.

<h2>Tying The X Caddis Step 3 of 7</h2>Dub a tapered abdomen up to the starting point. Clip the zelon shuck to about a shank length long.

<h2>Tying The X Caddis Step 4 of 7</h2>Cut, clean and stack a clump of deer hair. Measure the hair against the hook so it is equal to one shank length.

<h2>Tying The X Caddis Step 5 of 7</h2>Cut the hair to one shank length long and place the butt ends just behind the hook eye.

<h2>Tying The X Caddis Step 6 of 7</h2>Make two turns over the hair and pull toward you to tighten these wraps and flare the hair.

<h2>Tying The X Caddis Step 7 of 7</h2>Work the thread forward through the butt ends of the hair to the eye and whip finish. Clip the thread.